Rated 5 out of 5 stars

It works with Fx 3.0.1. Just do the following:
1. open your minimizetotray.xpi with any archiver.
2. find install.rdf.
3. extract install.rdf.
4. open install.rdf with any text editor.
5. find em:maxVersion.
6. replace 2.0 (or something like it) with 3.1.0.
7. save install.rdf.
8. replace install.rdf in the archive with your new file.
9. install the extension.